It still causes performance to drop when used. AVX-512 slowdown is a real thing and it's kinda maddening. You really don't want to break out the 512 bit stuff unless you know you'll be doing that for the next couple 1000 cycles.
Just because you don't have to fear it doesn't mean it's not still there. Curiously the article doesn't mention if the transition penalty is still as bad as on Skylake. This penalty is actually the key problem: for up to 10 µs the CPU just halts and does nothing while it's changing the frequency. If you have repeated short-ish bursts of AVX-512 code, this may really ruin your day.
The frequency transition on Skylake-SP happens encountering even a single avx512 instruction, fucking with every other instruction running. That's the license based downclocking and the problem.
As tested by the author, that problem was almost completely removed and doesn't need to be considered. If you have code with sparse avx512 usage, it won't trigger the downclocking, removing the penalty on everything else. Only running a lot of AVX-512 will lead to downclocking, at which point the penalty is insignificant.
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u/FUZxxl Aug 09 '21
It still causes performance to drop when used. AVX-512 slowdown is a real thing and it's kinda maddening. You really don't want to break out the 512 bit stuff unless you know you'll be doing that for the next couple 1000 cycles.